1. Field of the Invention
This invention relates to liquid formulations containing a sulfosuccinic acid diester, to a process for their production and to the use of these liquid formulations as wetting agents and emulsifiers.
2. Statement of Related Art
Diesters of sulfosuccinic acid have been known for many years as effective wetting agents. In the course of the changeover from systems containing organic solvents to water-based systems, this class of compounds has acquired increasing significance. The synthesis of the diesters would appear at first glance to be very simple, as reflected in early patent literature. Thus, U.S. Pat. No. 2,176,432, U.S. Pat. No. 2,416,254 and U.S. Pat. No. 2,028,091 describe the production of sulfosuccinic acid diesters by methods which cannot be carried out on an industrial scale and which are not economically justifiable.
GB-PS 1,050,578, FR-PS 1 573 080, GB-PS 760,121 and U.S. Pat. No. 2,879,214 describe the production of liquid formulations of sulfosuccinic acid diesters on an industrial scale. However, it is clear from these documents that the reaction is characterized by several special features and that controlled measures have to be taken to influence the reaction. It is less the synthesis of the maleic acid diesters which are used as raw materials in the synthesis of sulfosuccinic acid diesters than the sulfitation of this raw material to the sulfonated component which is regarded as problematical. A review of the sulfitation of maleic acid diesters can be found in Anionic surfactants, Part II, W. M. Linfield, M. Dekker, 1976, pages 406 et seq. and in Handbook of surfactants, M. R. Porter, Blackie, 1991, pages 107 et seq.
It is clear from the literature that the problem with sulfitation lies in the reaction of a hydrophobic diester with a water-soluble HSO.sub.3- and in the formation of gel phases. According to various references, the reaction is very slow and, after an initial phase, is clearly exothermic and difficult to control. To be able to carry out the reaction, it is recommended in GB-PS 1,050,578 to use an initiator. The initiator may be a surfactant, more particularly the sulfosuccinic acid diester to be produced, or a solvent, such as ethanol. At the same time, an organic solvent, such as mineral oil, white oil, paraffin or an aromatic solvent, is used. FR-PS 1 573 080 describes the addition of the sulfite solution to enable the exothermic nature of the reaction to be better controlled. This document also mentions the possibility of carrying out the reaction under pressure which allows reaction temperatures above the boiling point of the mixture and prevents the loss of sulfite through evaporation tion of SO.sub.2. A short reaction time is mentioned as an advantage. U.S. Pat. No. 2,879,214 recommends the exposure of the reaction mixture to UV light to accelerate the reactions. The use of ethanol is described explicitly or without any particular references in FR-PS 1 573 080, in GB-PS 760,121 and in U.S. Pat. No. 2,879,214.